Bump snok/container-retention-policy from 2.1.2 to 2.2.1

Bumps [snok/container-retention-policy](https://github.com/snok/container-retention-policy) from 2.1.2 to 2.2.1.
- [Release notes](https://github.com/snok/container-retention-policy/releases)
- [Commits](snok/container-retention-policy@v2.1.2...v2.2.1)

---
updated-dependencies:
- dependency-name: snok/container-retention-policy
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

.github/workflows/prune-untagged-images.yml

@@ -23,7 +23,7 @@ jobs:
           - met-upperair
     steps:
       - name: Delete image
-        uses: snok/container-retention-policy@v2.1.2
+        uses: snok/container-retention-policy@v2.2.1
         with:
           image-names: metexpress/development/${{ matrix.app }}
           cut-off: 1 day ago MST